Now the blockstate json (resources/assets/firstmod/blockstates/simpletexturedblock.json):
(Please note that "simpletexturedblock.json" is the exact same name as the name set in your unlocalizedName and RegistryName above!)

The model itself (resources/assets/firstmod/models/block/simpletexturedblock.json):
(Please note that "simpletexturedblock.json" is the exact same name as the name set in your unlocalizedName and RegistryName above!)